It has been a common problem in the electrical utility industry to analyze dissolved gas concentrations in the electrical insulating oil of the so-called oil-immersed transformers. If faults occur within the transformer, such as from arcing, pyrolysis (overheating), corona discharge, and the like, certain gases are evolved into the oil supply. Fault gases that have been recognized in the past as identifying specific transformer faults have included ethane, ethylene, acetylene, methane, carbon dioxide, carbon monoxide, hydrogen and oxygen. Increases in fault gas concentration levels in the transformer oil can be measured to identify specific types of malfunctions as well as their severity. If a transformer problem is allowed to go undetected, the electrical utility can suffer extremely high replacement costs. Worker safety also is a major concern because of the explosive nature of certain fault gases, such as acetylene, hydrogen and carbon monoxide any of which may react with oxygen. Reliable detection of such fault gases before the concentration levels become dangerously high is an objective of the present invention.
Electrical utilities normally send a person to the transformer site to regularly collect oil samples and return them to a laboratory for testing the fault gas concentration levels. If a particular fault gas exceeds a certain level, it will be a warning of a particular fault which can then be corrected.
The present invention is based on a recognition that periodic checking of oil samples from large power transformers may not occur often enough to detect a problem until it is too late for reasonable corrective action. The present invention solves the problem by providing continuous on-line fault gas concentration data from the transformer site that can be constantly monitored to detect a problem the moment it arises.
In the past, gas chromatograph techniques have been used to measure fault gases extracted from oil samples. One standard technique for detecting fault gases in transformers has involved use of a mercury Toepler pump method of gas extraction. Such analytical techniques require expensive laboratory equipment, and the complete analysis is time consuming. There is also a danger in working with large amounts of mercury in glassware that is under high vacuum in order to extract the gases from the oil sample. In addition, oil samples containing very low levels of certain gases cannot provide enough gas to perform accurate chromatographic analyses. Handling of oil samples also can result in the loss of certain volatile gases such as hydrogen.
In another prior method of measuring fault gases, an oil stripper column is inserted in the oven chamber of a gas chromatograph. An argon carrier gas is used for the stripping process. The laboratory equipment is expensive, and the process to analyze gas samples is slow. Since gas solubilities vary with temperature, the extraction efficiencies, which are different at the test temperature compared to room temperature, contribute to inaccuracies in measurements for certain gases. Discrepancies for certain gas measurements have been found when test results from this method are compared with the mercury Toepler pump extraction method.
Thus, there is a need for a transformer fault gas detection system which can analyze fault gases with extreme accuracy, and in which such analysis is accomplished at a reasonably low cost and without the delay times caused by oil sample extraction and analysis and the complex equipment associated with commonly used fault gas analytical techniques.